Method of removing the pattern from a thin shell investment mold



INVESTMENT MOLD I PA 7' 72 1? /V N. J- GRANT EMOVING THE PATTERN FROM May 12, 1964 METHOD OF R A THIN SHELL Filed Aug. 26, 1959 A rraexvsv United States Patent York Filed Aug. 26, 1959, Ser. No. 836,573 6 Claims. (Cl. 22-41%) This invention relates to the art of precision casting of metal articles and particularly to the method for removing patterns from shell investment molds or thin-walled, onepiece molds of ceramic refractory materials.

Thick-walled investment molds suitable for precision casting have, in the past, been made by the so-called lost-wax method which comprises coating a pattern of wax, plastic or other expendable material with a slip of finely divided ceramic refractory material, sprinkling or stuccoing the coating with coarse refractory material and embedding the coated pattern in a thick refractory investment within an outer steel container or flask, after which the wax is removed by slowly heating the entire mold for several hours in an inverted position to melt the wax. Such a mold, of necessity, is heavy and cumbersome since it must be thickly invested to resist the large initial expansive force of the wax pattern, amounting to about 4%-9% by volume, when heated to its melting temperature. A mold of such thicknes has poor heat conductivity and, when a molten metal is cast into the cavity left by the removal of the wax pattern, the inner shapedcontrolling surface layer of the mold is subjected to thermal shock and expansion forces which cause cracks and deformation of its inner surface layer. The removal of the investment material from the casting is usually quite difiicult.

To avoid these diificulties, it has recently been pro posed to make a thinly invested shell mold by utilizing for the pattern frozen mercury, which has a very small thermal expansivity near its melting point and which can be melted and removed without breaking the mold. By thus using a pattern having insuflicient thermal expansivity at and below its melting point to cause breakage of the coating it is, therefore, possible to form a shell investment mold having a wall thickness of about to A" which can be used for precision casting without being invested in a thick supporting refractory mask.

The use of a frozen mercury pattern necessitates carrying the various coating steps at a temperature below 39 C. and requires the use of a non-aqueous slip or one having a suitably low freezing point. The difiiculties entailed by such temperature limitations are manifold and the process, although it will produce a highly useful mold,

is quite expensive and diflicult to perform.

Another method which has been proposed for precision casting consists of forming a thin-walled shell investment on a conventional Wax-like pattern and preventing the expansion of the pattern from breaking the walls of the mold by dipping the pattern and mold into a liquid maintained at a temperature equal to or above the melting point of the pattern material. This causes the hot liquid to be forced under pressure into the porous Walls of the mold and thereby contact the pattern at the interface between the pattern and the mold which softens the pattern material before the body of the pattern has been heated high enough for it to exert expansion stresses on the mold sufiicient to break it.

However, this method is not altogether satisfactory because the liquid weakens the mold structure and can cause undesirable discontinuities in the casting surface of the mold. Furthermore, it also requires that the liquid so introduced into the mold be volatilized before the mold can be sintered.

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Another method which has been proposed for removing the pattern from a thin-walled shell investment is to subject the mold and pattern contained therein to the vapors of solvents for the wax pattern material. This method has the disadvantages that such vapors are usually toxic and combustible and the use of such solvents increases the cost of the dewaxing operation and requires that condensed vapors be revolatilized'before the mold can be fired.

The principal object of this invention is to provide a simplified method for the removal of a pattern from a shell investment mold.

A further object of this invention is to provide a method for the removal of a pattern from a shell investment mold which does not require the use of a pattern solvent or liquid heat-transfer media.

A still further object of this invention is to provide a method of removing a pattern from a shell investment mold which does not require drying of the mold after the removal of the pattern material.

A still further object of this invention is to provide a method of making a shell investment mold which is amenable to combining the pattern-removal step and the mold-sintering step into a single step.

In the drawing accompanying this application and forming a part thereof:

FIG. 1 is a side view of a pattern or core to be used for .the production of a shell investment mold with gate and riser according to this invention; and

FIG. 2 is a sectional view of a shell investment mold with gate and riser, made in accordance with this invention, after the removal of the pattern.

I have now found that the above objectives can be ob tained by a method of removing the wax pattern which comprises introducing a pattern and shell investment mold into an atmosphere consisting essentially of inert gases which is maintained at a sufficiently high temperature in order that the heat is transferred, primarily by radiation, through the mold to the pattern at its interface with the mold so that the pattern melts at the interface before the body of the Wax has increased in temperature causing expansive forces to break the mold walls. I

In carrying out the new process according to themeferred embodiment of the invention an accurately dimensioned pattern (FIG. 1) of the article to be cast, including its gate and riser, is coated by repeated dipping ina slip or slurry containing powdered fused or amorphous silica or, preferably, a high silica glass of the type disclosed in Patent No. 2,106,744 having a silica content of 94% or higher and pulverized to pass a screen having 325 meshes per linear inch. Although suitable procedures for producing a coating of requisite thickness on the wax pattern are well known, the following procedure is particularly advantageous for the practice of the present invention. V For the primary coat the slip preferably contains 97.5 parts by weight of the powdered silica or high silica glass and 2.5 parts of equally fine ball clay to provide plasticity and bond, suspended in the requisite amount of a suitable aqueous vehicle which preferably contains a small amount of a coagulent such as tartaric acid, a small amount of a water-soluble gun such as gum arabic to impart green strength, a small amount of a suspended colloidal silica for a refractory bond, anda small amount of a wetting agent such as sodium dioctylsulfosuccinate. Although the proportions of such ingredients in the vehicle are not particularly critical, the following amounts have been found suitable; 1.5 cc. of a 50 weight percent aqueous solution of tartaric acid, 15 cc. of a 10 weight percent aqueous solution of gum arabic, 5 cc. of a 35 Weight percentaqueous suspension of colloidal silica, 0.1 cc. of a weight percent aqueous solution of sodium 3 dioctylsulfosuccinate and 9.5 cc. of distilled or softened water per 100 parts of the glass and clay.

After being dipped in the slip, the coated wax pattern is allowed to drain for a few minutes and is sprinkled or stuccoed with pulverized, fused silica or high silica glass, preferably having a grain size between 35 and 50 mesh. It is then dried in air for about one hour at room temperature.

In order to build up a coating of suitable thickness, the wax pattern with its dried primary coat is subsequently dipped and dried 3 to 9 times, preferably times, but in forming the subsequent coats the balled clay is omitted from the slip and 100 parts of the powdered fused silica or high silica glass are used, while the tartaric acid and gum arabic are omitted from the vehicle and a small amount, 0.5 cc. of a 1.5 weight percent aqueous solution, of metal cellulose is added instead. A coarser stucco, between 25 and 35 mesh, is used on the second and succeeding coats up to the last coat and, between 10 and mesh is used on the last coat. The finally coated pattern or mold is allowed to dry thoroughly in air at room temperature, preferably for about 16 hours.

According to the present invention the pattern may then be removed from the mold by inserting the mold and pattern into a furnace atmosphere consisting essentially of inert gas, preferably air, maintained at a temperature sufficiently high to cause the pattern to liquefy at the interface with the mold before the body of the pattern has expanded sufficiently to cause the mold walls to crack. While the actual furnace temperature will vary to some extent with the melting point of the pattern material and to a lesser extent with the thermal coefiicient of volume expansion of the wax utilized, 1 have found that a temperature of 325 C. is the minimum temperature which can be utilized with a wax pattern having a melting point of 75 C. and a volume expansion coefficient between C. and 75 C. of 6%. Thus it can be seen that a furnace atmosphere maintained at a temperature of at least 250 C. in excess of the melting temperature of the wax is sufficient.

Furthermore, I have found that while an atmosphere temperature of 250 C. in excess of the melting temperature of the wax is sufficient to produce a mold which is usable for casting articles which do not have close dimensional tolerances or which can be readily machined to close tolerances, that a temperature at least 675 C. in excess of the melting point of the wax is required to produce a mold which comforms almost exactly to the dimensions of the pattern. This appears to be the result of the fact that the mold has sufficient strength to withstand a small amount of deformation caused by small expansive stresses of the pattern, but the higher temperatures prevent even slight stresses by the pattern material on the mold. Thus, for the preferred pattern material and the waxes most commonly utilized for preparation of molds of this type, a minimum, initial de-waxing temperature of 750 C. is preferred.

Since it is desirable also to strengthen the green refractory shell by firing it to maturity as soon as possible after the removal of the wax pattern, the most desirable method for the removal of the wax pattern is to place the mold and pattern in a furnace maintained at a sufficiently high temperature whereby the refractory shell is matured or sintered and substantially strengthened and at the same time the wax is liquefied, and probably to some extent vaporized. Thus it is desirable to introduce the inverted mold suddenly into a furnace preheated at a temperature of 900 C. to 1150 C. and preferably about 1050 C. Still higher temperatures, if used, cause no improvements in the results, although they generally cause no impairment, they should not be so high as to cause incipient fusion or shrinkage of the refractory with consequent loss of accuracy of dimensions. Temperatures below 900 C. are satisfactory for removing the pattern material, but proper firing of the refractory shell requires that such low temperatures be subsequently increased.

When the mold is introduced into the preheated furnace, it is preferably inverted upon an open grid of refractory material such as a nickel-chromium alloy having a wide enough mesh to permit ready passage of the wax therethrough while providing stable support for the mold. The furnace may be of any conventional type and heated by gas, electricity, or the like. The furnace may also be provided with a suitable bottom opening such as removable section, the momentary removal of which permits most of the wax to be salvaged. Under the furnace conditions suitable for the practice of the present invention, most of the wax is removed in about 1 to 10 minutes. Since removal of the bottom section of the furnace may allow its temperature to fall rapidly, particularly when it is large enough to accommodate a large number of molds simultaneously, it is desirable that this interval be as brief as possible. It is also desirable, after the replacement of the bottom section and recovery of the temperature in the furnace, that heating of the mold be continued long enough, say about 20 minutes, to completely oxidize and burn out any wax remaining in the mold.

While practically any ceramic refractory material may be utilized for making shell investment molds according to the present invention, powdered fused silica and the above-mentioned high silica glass are particularly useful because molds made therefrom are capable of withstanding severe thermal shock on account of their low thermal expansion coefficients. Finely divided zircon, alumina, calcined fire cla and sillimanite have successfully been substituted in the above-described slip and used in the above-described procedure to produce shell molds suitable for the precision casting of metal according to this invention.

While the known waxes heretofore used in the practice of the lost-Wax process are preferred for carrying out the procedure of the present invention, it is possible, though less satisfactory, also to utilize other thermoplastic substances of suitably low melting temperatures known in the prior practice. The term wax in the appended claims includes such thermoplastic substances.

The atmosphere in the furnace can comprise any gas which is a gas at room temperature so that it will not condense on the mold when the mold is inserted in the furnace and also that will not react with the pattern material so as to require subsequent processing of the atmosphere to separate the Wax therefrom. Suitable gases and mixtures thereof which are intended to be included within the meaning of the term inert gas as used herein are air, nitrogen, argon, neon, helium, carbon dioxide, and even the gaseous products of combustion of fuel used to fire the furnace. Air, of course, is preferred because of its availability.

This application is a continuation-in-part of my application Serial No. 634,520, filed January 16, 1957, now abandoned.

What is claimed is:

1. The method of removing a wax pattern from a shell investment mold formed thereon, said mold having a wall thin enough to allow rapid transfer of heat therethrough and not sufiiciently thick to resist breakage from the force which would result from expansion of said wax pattern upon slow heating thereof, which comprises exposing the mold and pattern to a surrounding atmosphere consisting essentially of inert gas maintained at a temperature at least 25 0 C. in excess of the melting point of the wax to liquefy the wax at its interface with the mold before the overall temperature of the wax body has substantially increased, and continuing the heating of the mold until the wax is completely removed.

2. The method of making a shell investment mold which comprises repeatedly applying and drying a slip of finely divided ceramic refractory material on a wax pattern having accurate predetermined dimensions to form a coating on said pattern, alternately sprinkling said coating between applications With coarser particles of a ceramic refractory material to form a wallthin enough to allow rapid transfer of heat therethrough and not sufiiciently thick to resist breakage from the force which would re sult in expansion of said wax pattern upon slow heating thereof, drying the refractory shell on its wax pattern and thereafter introducing them without further investment into an atmosphere consisting essentially of inert gas preheated to a temperature at least 250 C. in excess of the melting point of the wax to liquefy the wax at the interface before the volume of the Wax body has substantially increased by thermal expansion, and continuing the heating to sinter the refractory coating and completely remove the Wax.

3. The method of claim 2 in which the temperature of the atmosphere is at least 750 C.

4. The method of claim 2 in which the temperature of the atmosphere is at least 900 C. to 1150 C.

5. The method of claim 2 in which the ceramic refrac- 6 tory material comprises at least 97.5% of a high silica glass containing over 94% SiO I 6. The method of claim 2 in which the ceramic refractory material comprises at least 97.5% of amorphous silica.

References Cited in the file of this patent UNITED STATES PATENTS 2,491,096 Feagin Dec. 13, 1949 2,518,040 Mann Aug. 8, 1950 2,759,232 Demeter et a1. Aug. 21, 1956 2,871,528 Schilling Feb. 3, 1959 2,932,864 Mellen et a1 Apr. 19, 196 0 FOREIGN PATENTS 163,941 Australia May 20, 1954 OTHER REFERENCES Glascast, Precision Shell Molds Made by the Lost Wax 20 Process, by Corning Glass Works, Bulletin GC-3, July 

1. THE METHOD OF REMOVING A WAXY PATTERN FROM A SHELL INVESTMENT MOLD FORMED THEREON, SAID MOLD HAVING A WALL THIN ENOUGH TO ALLOW RAPID TRANSFER OF EHAT THERETHROUGH AND NOT SUFFICIENTLY THICK TO RESIST BREAKAGE FROM THE FORCE WHICH WOULD RESULT FROM EXPANSION OF SAID WAX PATTERN UPON SLOW HEATING THEREOF, WHICH COMPRISES EXPOSING THE MOLD AND PATTEN TO A SURROUNDING ATMOSPHERE CONSISTING ESSENTIALLY OF INERT GAS MAINTAINED AT A TEMPERATURE AT LEAST 250*C. IN EXCESS OF THE MELTING POINT OF THE WAX TO LIQUEFY THE WAX AT ITS INTERFACE WITH THE MOLD BEFORE THE OVERALL TEMPERATURE OF THE WAX BODY HAS SUBSTANTIALLY INCREASED AND CONTINUING THE HEATING OF THE MOLD UNTIL THE WAX IS COMPLETELY REMOVED. 